Injection drug use is a serious public health problem, leading to blood-borne infections including HBC, HCV, and HIV. In spite of advances in the prevention of HIV/AIDS and hepatitis, injection drug use continues to contribute to new infections both directly through the sharing of injection equipment, and indirectly through sexual transmission from injection drug users (IDUs) to non-IDU sex partners. Behavioral interventions with IDUs are an important component of a comprehensive HIV prevention strategy. Enhanced interventions with IDUs have shown only modest effects over and above standard interventions. The key to improving interventions with IDUs may lie in understanding variations in responses to these interventions. Latent variable approaches (also known as mixture modeling) can be used to identify subgroups of individuals with similar response patterns. These approaches are often referred to as person-centered, in contrast to the traditional variable-centered approaches. To better understand how HIV prevention interventions influence injecting and sexual behavior of IDUs, we will conduct secondary analyses of data from the Third Collaborative Injection Drug Users Study / Drug Users Intervention Trial (CIDUS-3/DUIT). This study recruited young injection drug users in five U.S. cities to test a novel peer education intervention (PEI) against an attention control on changes in HIV/HCV risk behavior. We will use mixture modeling approaches, including latent class analysis, growth mixture modeling, and latent transition analysis, to analyze program outcomes. Our aim is to identify specific subgroups of IDUs for whom the intervention was most beneficial. Moreover, we seek to identify predictors of group membership which would enable us to assign individuals to the intervention that is most likely to have an impact. The proposed analyses will increase our understanding of the effects of the standard and enhanced interventions on injection and sexual risk behavior, and lead to recommendations for improving intervention efforts with injection drug users. The results may also have wider implications for strategizing prevention interventions with other populations. PUBLIC HEALTH RELEVANCE: Injection drug use facilitates blood-borne infectious diseases with significant morbidity and mortality, including HIV and hepatitis C. The CIDUS 3/DUIT Peer Education Intervention has the potential to significantly reduce injection-related risk behavior and sexual risk behavior among injection drug users. The proposed analyses will provide information to improve the implementation of this program and other HIV prevention interventions.